Our proposal involves the high resolution structural determinations of modified forms of the restriction endonuclease EcoRV bound to DNA using mutants of EcoRV and modified DNA substrates. Since our overall goal is the correlation of structural perturbations between modified and wild type complexes to functional parameters such as binding affinity and catalysis, very high resolution (2.0 E or better) structures are required. EcoRV has already been solved in several crystal forms, and therefore no heavy atom reagents will be required for the structure solution, which can be solved instead by molecular replacement.